1. Field of the Invention
The present invention relates to a polarisation independent optical phase modulator. Such a modulator may be used to provide adaptive phase modulation to allow active manipulation of light, for instance for applications such as interferometry, deflection such as beam steering, imaging such as forming a Fresnel lens, and analogue modulation of diffracted light, for instance to provide a diffraction-based projection display.
2. Description of the Related Art
Eschler et al, "Liquid Crystal Light Valves for Schlieren Optical Projection", Displays, Volume 16, page 35, 1995 discloses a polarisation independent optical phase modulator comprising an electrooptic modulator, a retarder or static waveplate, and a mirror. The use of such a modulator in a diffraction-based projection system is also disclosed. The electrooptic modulator comprises a nematic liquid crystal device acting as a phase modulator. The nematic liquid crystal is untwisted and has antiparallel alignment so that, when an electric field is applied across the liquid crystal layer, the molecules tilt in a plane perpendicular to the layer so as to vary the retardation of light passing through the layer.
The waveplate is a static quarterwave plate whose optic axis is aligned at 45 degrees to the alignment direction of the nematic liquid crystal. The quarterwave plate is disposed between the electrooptic modulator and the mirror.
Operation of the phase modulator is illustrated in FIGS. 1 and 2 of the accompanying drawings. The reflective light path has been shown "unfolded" about fold plane FP so as to illustrate the operation more clearly. An arrow 1 illustrates the direction of light propagation through the device. A single arbitrary polarisation state of an unpolarised light beam is represented by orthogonally polarised components 2 and 3 having a phase difference .phi. between them. The light passes through the electrooptic modulator shown at 4 and the vertically polarised component 2 is delayed by 2.pi..DELTA.nd/.lambda. relative to the horizontally polarised component, where .DELTA.n is the birefringence of the electrooptic material of the modulator 4, d is the thickness of the modulator 4, and .lambda. is the wavelength of the light.
The mirror and quarterwave plate act as a static halfwave plate 5 whose optic axis is aligned at 45 degrees to the alignment direction of the modulator 4. The polarisations of the components 2 and 3 are therefore "reflected" about the optic axis as a result of passage through the halfwave plate formed by the quarterwave plate and the reflector. The light then passes again through the electrooptic modulator shown at 6, where the vertically polarised component 3 has its phase delayed by 2.pi..DELTA.nd/.lambda.. Thus, the components 2 and 3 both experience the same phase delay and remain orthogonal, but are reflected about the axis of the waveplate. Thus, although the electrooptic modulator provides polarisation dependent phase modulation, the change in polarisation produced by the halfwave plate 5 causes both polarisation components to experience the same phase modulation. The device therefore acts as a polarisation independent optical phase modulator. In particular, unpolarised incident light can always be resolved into a linear sum of polarisation states, each described by two orthogonally polarised beams. As described above, the device phase modulates any arbitrary polarisation state independently of what that state is so that the device modulates all states by the same amount. Therefore it will operate on unpolarised light.
In this known phase modulator, the electrooptic modulator, the quarterwave plate, and the reflector are embodied as separate optical elements. The device is arranged as a plurality of individually controllable picture elements (pixels) so as to provide spatial phase modulation, for instance resulting in a diffractive optical device whose diffraction properties are controllable. However, because of the effects of parallax between the separate optical elements, relatively large pixels are required in order to permit a usable range of incident angles. This problem of the known device is illustrated in FIG. 2 of the accompanying drawings. Where the phase modulator is used with an optical system 7, for instance forming part of a projection display, the orthogonal polarisation components are imaged into different planes. The polarisation component 2 is optically modulated by the electrooptic modulator at the position 4 whereas the component 3 is modulated at the position 6. The images of the two orthogonal polarisations are therefore formed in the distinct image planes shown at 8 and 9.
U.S. Pat. No. 3,912,369 discloses a reflective liquid crystal display having a liquid crystal cell, a quarterwave plate, and a reflector. The liquid crystal cell, the wave plate and the reflector are provided as separate elements, with the wave plate and reflector being disposed outside the liquid crystal cell.
A spatial light modulator using ferroelectric liquid crystal technology is disclosed in a paper entitled "Diffractive Ferroelectric Liquid Crystal Shutters for Unpolarised Light" by M. J. O'Callaghan and M. A. Handschy, Optics Letters, Volume 16 No. 10, May 1995, pages 770 to 772. The spatial light modulator disclosed in this paper is switchable between a first state in which it transmits incident light and a second state in which it acts as a phase diffraction grating.
Another spatial light modulator is disclosed in a paper entitled "Improved Transmission in a Two-Level, Phase Only, Spatial Light Modulator" by M. A. A. Neal and E. G. S. Page, Electron. Lett. 30 (5) pages 465-466 1994. This paper discloses a spatial light modulator which is switchable between a transmissive mode and a diffractive mode in which alternate strips of the modulator rotate unpolarised light by plus and minus 45 degrees and an associated halfwave retarder further rotates all the polarisation components of the light so as to provide phase-only modulation.
FIGS. 3 and 4 of the accompanying drawings show a reflection-mode diffractive spatial light modulator (SLM) of the type disclosed in GB 9611993.8 (publication No. 2 313 920). The SLM comprises a rectangular array of rectangular or substantially rectangular picture elements, only one of which is shown in FIGS. 3 and 4. The SLM comprises upper and lower glass substrates 11 and 12. The upper substrate 11 is coated with a transparent conducting layer of indium tin oxide (ITO) which is etched to form elongate interdigitated electrodes 13. The electrodes 13 are covered with an alignment layer 14 for a ferroelectric liquid crystal material.
A combined mirror and electrode 15 is formed on the glass substrate 12 and a static quarterwave plate 16 is formed on the silver mirror and electrode 15. The thickness of the plate 16 is controlled so that it acts as a quarterwave plate for a predetermined bandwidth in the visible spectrum, for instance centred about 633 nanometers.
A further alignment layer 17 is formed on the quarterwave plate 16. The substrates 11 and 12 are then spaced apart and stuck together so as to form a cell which is filled with the ferroelectric liquid crystal material to form a layer 18. The spacing provides a layer of ferroelectric liquid crystal material which provides a halfwave of retardation so that the liquid crystal layer acts as a halfwave retarder whose optic axis is switchable as described hereinafter.
For each pixel, the electrode 15 acts as a common electrode which is connectable to a reference voltage line, for instance supplying zero volts, for strobing data to be displayed at the pixel. Alternate ones of the elongate electrodes 13 are connected together to form first and second sets of parallel interdigitated electrodes which are connected to receive suitable data signals. Each pixel is switchable between a reflective state and a diffractive state as described hereinafter.
FIG. 5 of the accompanying drawings illustrates diagrammatically the operation of adjacent strips of the pixel shown in FIGS. 3 and 4 when the pixel is in the diffractive mode. The optical path through each pixel is folded by reflection at the mirror 15 but, for the sake of clarity, the path is shown unfolded in FIG. 5. The SLM acts on unpolarised light, which may be split into components of orthogonal polarisations for the sake of describing operation of the SLM. One of the component polarisations is shown at 20 in FIG. 5 and is at an angle -.phi. with respect to a predetermined direction 21.
Voltages which are symmetrical with respect to the reference voltage on the electrode 15 are applied to the first and second sets of alternating interdigitated electrodes 13a and 13b. Thus, ferroelectric liquid crystal material strips 18a and 18b disposed between the electrodes 13a and 13b and the electrode 15 have optic axes aligned at angles of -.theta. and +.theta., respectively, with respect to the direction 21, where .theta. is preferably approximately equal to 22.5 degrees.
Each strip 18a of ferroelectric liquid crystal material acts as a halfwave retarder so that the polarisation of the light component leaving the strip 18a is at an angle of .phi.-2.theta. with respect to the direction 21. The light component then passes through the static quarterwave plate 16, is reflected by the mirror 15, and again passes through the static quarterwave plate 16, so that the combination of the quarterwave plate 16 and the mirror 15 acts as a halfwave retarder whose optic axis is parallel to the direction 21. The polarisation direction of light leaving the quarterwave plate 16 and travelling towards the ferroelectric liquid crystal material is "reflected" about the optic axis of the quarterwave plate and thus forms an angle 2.theta.-.phi. with respect to the direction 21. The light component then again passes through the strip 18a of ferroelectric liquid crystal material so that the output polarisation as shown at 24 is at an angle of .phi.-4.theta. with respect to the direction 21. Thus, for each input component of arbitrary polarisation direction -.phi., the optical path through the SLM via each of the strips 18a of ferroelectric liquid crystal material is such that the polarisation direction is rotated by -4.theta.. This optical path therefore rotates the polarisation of unpolarised light by -4.theta., which is substantially equal to -90 degrees.
Each strip 18b of ferroelectric liquid crystal material acts as a halfwave retarder and rotates the polarisation direction to .phi.+2.theta.. The fixed halfwave retarder formed by the combination of the quarterwave plate 16 and the mirror 15 rotates the direction of polarisation of the light component so that it makes an angle of -2.theta.-.phi. with respect to the direction 21. The final passage through the strip 18b rotates the polarisation direction to .phi.+4.theta. as shown at 25. Light of the orthogonal polarisation has its polarisation rotated in the same way. Thus, unpolarised light passing through the strips 18b has its polarisation rotated by +4.theta., which is substantially equal to +90 degrees.
Light reflected through each of the strips 18b is out of phase by 180 degrees with respect to light passing through each of the strips 18a when the electrodes 13b and 13a are connected to receive data signals of opposite polarity. In this state, the pixel acts as a phase-only diffraction grating and the pixel operates in the diffractive mode. Because of the bistable characteristics of ferroelectric liquid crystals, it is necessary only to supply the data signals in order to switch the strips 18a and 18b to the different modes illustrated in FIG. 5.
In order for the pixel to operate in the reflective mode, it is necessary to switch either or both sets of strips 18a and 18b so that their optic axes are parallel. Unpolarised light incident on the pixel is then substantially unaffected by the ferroelectric liquid crystal material and the quarterwave plate 16 and is subjected to specular reflection by the mirror and electrode 15. Each pixel is therefore switchable between a transmissive mode, in which light is specularly reflected or "diffracted" into the zeroth diffraction order, and a diffractive mode, in which light incident on the pixel is diffracted into the non-zero diffraction orders.
Such a diffractive SLM can be used with unpolarised light and provides increased optical modulation efficiency compared with SLMs which require polarised light.
FIG. 6 of the accompanying drawings illustrates a projection display using an SLM 30 of the type shown in FIGS. 3 and 4. The SLM 30 is illuminated by an unpolarised light source 31 via a mirror 32. A projection optical system 33 projects an image displayed by the SLM 30 onto a screen 34.
Light from the light source 31 is reflected by the mirror 32 so as to be incident normally on the SLM 30. Each pixel which is in the reflective mode reflects the incident light normally back to the mirror 32 so that the reflected light is not projected by the system 33. Thus, a "dark" pixel is imaged on the screen 34 by the system 33. Each pixel in the diffractive mode deflects the incident light into the non-zero diffraction orders, mainly into the positive and negative first orders as illustrated by light rays 35 and 36. The light from each such pixel is thus imaged to a "bright" pixel on the screen 34.